Emergency shutdown operation of recovery boilers



A. PARKIN Oct. 1, 1968 EMERGENCY SHUTDOWN OPERATION OF RECOVERY BOILERSI 4 Sheets-Sheet l UVVE/W'Of AA z/m/ PAW/VA BY M W Filed April 9, 1965Pmae ART A. PARKIN Oct. 1, 1968 EMERGENCY SHUTDOWN OPERATION OF RECOVERYBOILERS 4 Sheets-Sheet 2 Filed April 9, 1965 PRIOR ART PRIOR ART PZ/ORART ml/flw'ak flaw PARK/N By a Oct. 1, 1968 A. PARKIN 3,403,642

EMERGENCY SHUTDOWN OPERATION OF RECOVERY BOILERS Filed April 9, 1965 4Sheets-Sheet 5 RH 4- w- 63% C1 X Z i U l] LU u n i i I. 51 i U i l i" O6/ GI m at $67. nun nun nhuhn I 6} 26 I l Kiri 1 n 1 t m um an M l m ()5nun mm H 49)?) nunm Oct. 1, 1968 A. PARKIN 3,403,642

EMERGENCY SHUTDOWN OPERATION OF RECOVERY BOILERS 4 Sheets-Sheet 4 FiledApril 9, 1965 United States Patent G 3,463,642 EMERGENCY SHUTDQWNOPERATION OF RECOVERY BOILERS Alvin Parkin, RR. 2, Campheil River,British Columbia, Canada Filed Apr. 9, 1965, Ser. No. 446,937 lairnspriority, application Canada, Aug. 7, 1964,

998,954 6 Claims. (Cl. 1l07) This invention relates to improvements inthe manner of construction and method of operation of a recovery boilerof the type employed for example in the so-called kraft sulphate pulpprocess in the paper industry.

More specifically, the invention is directed towards a recovery boilerwhich constitutes a black liquor recovery unit. In the production ofpulp, wood chips are cooked in digcstors with a white liquor of whichthe active chemicals are sodium hydroxide and sodium sulphide. Thisaction separates the lignin from the cellulose of the wood, the ligninbeing the binder material that holds the cellulose fibres together. Thecellulose, upon separation from the lignin, provides the pulp for themanufacture of paper.

The residual black liquor containing the spent chemicals and thedissolved lignin is collected, because it is possible to recover thechemicals from this liquor by burning off the lignin. Such recovery ofthe chemicals is necessary, in order to make the process economicallyfeasible, and this is the prime purpose of a recovery boiler. However, agreat deal of heat is generated in the burning process and it has becomeconventional to convert this heat to steam for general use in the plant.

In order to provide a fuller understanding of the typical structure ofthis type of boiler, which is now extensively used by the industry,reference will now be made to the accompanying drawings, for whichpurpose a list of such drawings is first set out.

FIGURE 1 shows the main parts of a recovery boiler of the type referredto above, at least those parts relevant to the present considerations;

FIGURE 2 is a larger fragmentary view of the lower part of the boilershown in FIGURE 1, taken in central section;

FIGURE 3 is a section on the line IIIIII in FIG- URE 2; and

FIGURE 4 is a large scale fragmentary section on the line IV-IV inFIGURE 2.

The drawings so far listed represent a conventional boiler and hence theprior art to the present invention.

The structural aspects of the present invention are illustrateddiagrammatically and by way of example only in FIGURES 5 to 10, detailsof which are as follows.

FIGURE 5 is a view corresponding to FIGURE 2 showing modifications ofthe structure of the boiler in accordance with the present invention;

FIGURE 6 is a fragmentary section on the line VI-VI in FIGURE 5;

FIGURE 7 is a fragmentary section on the line VII- VII in FIGURE 5;

FIGURE 8 is a central sectional view of a lower portion of the modifiedboiler;

FIGURE 9 is a view of a typical burner for use in the arrangement ofFIGURE 8; and

FIGURE 10 is an enlarged view taken on the line XX in FIGURE 5.

The conventional boiler construction illustrated in FIG- URES l to 4comprises essentially a large, rectangular, vertically elongatedenclosure 10 hung from steel beams 11 beneath the roof of the building.The enclosure is formed by means of walls composed of an array ofvertically disposed tubes 12 (seen in detail in FIGURE 4) joinedtogether by webs 13. In addition to surrounding the interior of theboiler in this manner, some of the tubes 12 extend at 12a to form asloping floor for the boiler. In operation the tubes contain water, andthey are interconnected by headers shown only generally at 13a beneaththe floor of the boiler and also communicates with Water storage tanks14 and superheating and screen tubes 15 arranged in the upper part ofthe enclosure. The tubes 15 which are provided in addition to the tubes12 that form the bounding walls of the boiler are located generallyabove the central area where combustion takes place. Since the manner ofarrangement and operation of the water system of the boiler is en tirelyconventional and has not been modified by the present invention, nofurther description is believed necessary.

A forced draft fan is provided to blow air through ducts 21 and 22 whichin turn lead to ducts 23, 24 and 25 which surround the four walls of theboiler at different levels. As best seen in FIGURE 2, these ducts serveto supply a draft of air to a series of primary air ports 26, a seriesof secondary air ports 27 above the primary air ports 26, and a seriesof tertiary air ports 28 arranged at a still higher elevation in theboiler. In practice, these ports are formed by omitting short lengths ofweb 13 between the tubes 12 and bending short sections of such tubestowards each other to form openings communicating with the interior ofthe boiler. Apart from an access port 29, which is used when the boileris shut down and is closed during operation, the air ports 26, 27 and 28provide the only means of access for air to the interior of the boilerenclosure. An induced draft fan is also provided, and has beenillustrated diagrammatically at 30 in FIGURE 1. In practice theinstallation is normally more complex than has been shown, including acyclone evaporator where the flue gases are brought into contact withthe black liquor to increase the solids content thereof. Again, thesecomponents are in no way modified by the present invention and adetailed description of them would be irrelevant for the purposes of thepresent specification.

The wall tubes 12 are surrounded on the outside by a layer of thermalinsulation 31 and, below the line 32 which is referred to as the fullstud line, the inside faces of the tubes 12 are provided with projectingstuds 33 which serve at least temporarily to retain black liquor whichis sprayed onto them, to cake and ultimately drop down onto the charbed.

Prior to the boiler being placed in service a fire bed is formed abovethe lower tubes 12a, this floor being made of chrome dust which ishammered down into hard, brick-like form.

A spray 41 for the black liquor is provided in one of the walls of theboiler. A typical such black liquor spray is arranged to oscillate andto spray the black liquor all around the inside of the boiler, downtowards the floor and towards all parts of the wall for the liquor toadhere at least in part to the walls, and more particularly to the studs33 on the lower portions of the tubes 12. In addition to the blackliquor spray 41, of which there may be more than one, a series of oilburners 42 are arranged around the sides of the boiler. Typically, a fewsuch burners will be provided at a level above the primary ports 26, andsome will be located above the secondary ports 27.

When the boiler is started up, black liquor is sprayed onto the fire bed40 from the spray 41 and oil is sprayed in through the burners 42 andignited by any suitable means. The burning oil causes combustion of theblack liquor and, after operation for a short time, there builds up inthe centre of the boiler enclosure a roughly conical heap of burningmaterial which is represented diagrammatically in FIGURES l and 2 by thebroken line 44.

This heap of burning material is referred to as the char bed and willburn at a temperature typically somewhere within the range of l200 to1500" F., but sometimes in part as high as 1800 F. Once the fire hasthus been thoroughly started, the oil burners are turned off and nofurther oil is normally applied to the boiler. Indeed the burners arenormally withdrawn from the walls at this time. Combustion continuesfrom the black liquor which continues to be supplied from the spray 41.The char bed yields a liquor called smelt which contains the chemicalsto be recovered, the lignin having provided the source of heat and beinglargely converted to gas. In addition to combustion of the lignin, thesodium sulphate in the black liquor combines with carbon to producesodium sulphide, carbon monoxide and carbon dioxide. Sodium carbonate isalso formed from the lignin. The smelt is drained off through a duct 45for further treatment in a conventional manner in a hot water dissolvingtank 46.

The forced draft fan 20 and the induced draft fan 30 combine to maintaina continuous flow of air upwardly from the char bed through the boiler,thus maintaining and encouraging combustion at the char bed and removingthe products of combustion which thus heat the superheating and screentubes 15 in the upper part of the boiler, while the char bed itselfmainly imparts its heat to the water in the wall tubes 12.

The air supplied through the primary ports 26 is at a comparatively lowpressure in order to promote a reducing atmosphere in the burning massof char. The secondary ports 27 which are about 5 to 6 feet above theprimary ports 26 have air supplied through them at a slightly higherpressure, in order to promote burning of combustible materials risingfrom the glowing mass of the char bed, while the air supplied to thetertiary ports 28 is at a still higher pressure, being designed tofurther and complete the combustion process.

As has been mentioned, the oil burners 42 are not permanently fixed inthe unit. They are inserted through ports at approximately the level ofthe secondary ports 27, as and when they are needed. Normally theseburners 42 are only required for starting up the boiler, for operationunder adverse operating conditions .(as will be more specificallydescribed below), and for burning down the char bed when shutting downthe boiler. For shut down, the supply of black liquor is turned ofi andsufficient oil is supplied to ensure that the char bed is fullyconsumed. Typically, such a shut down operation takes from 4 to 6 hours.

When the boiler is operating normally, the char bed assumes the shape ofan irregular heap the sides of which are approximately on a level withthe primary ports 26, while its high points are approximately on a levelwith the secondary ports 27. Drying combustible material is continuouslyfalling on top of this heap, as the organic material is burnt off. Suchcombustible material comes directly from the spray 41 and also fallsfrom the side and back walls on which it has become partially dried andcaked as it runs down the faces of the wall tubes 12.

All the description of structure and operation which has so far beenprovided in this specification relates to conventional boilerconstruction and operation.

One of the principal difficulties in operating this type of boiler isthe danger of explosion. In recent years several men have lost theirlives or been seriously injured in pulp mill explosions, and extensivedamage to plant has been incurred. The danger of explosion arises fromleakage of water from the tubes some measure of which is virtuallyinevitable at some time or other. There are so many tubes, all subjectedto rigorous temperature conditions, that, as a practical matter, it isimpossible to operate for any length of time without tolerating acertain amount of water leakage. Serious leaks require immediate shutdown of the boiler, but naturally it is desirable to keep the number ofshut downs to a minimum. The operating time lost by a shut down issubstantial and this is particularly true in the case of an emergencyshut down when no time had been available for first consuming the charbed. If the boiler is allowed to cool with the char bed unconsumed, itssubsequent removal when cold is a lengthy, expensive and at timesdangerous undertaking.

It therefore becomes necessary to accept the fact that operation with acertain degree of water leakage onto the char bed is economicallyessential. Procedures have been worked out and are in operation forminimizing the danger resulting from water leaks. Moreover, at any timewhen the danger appears to be becoming excessive there is immediatelyput into effect an emergency shut down procedure. Notwithstanding theuse of the latter, a number of serious explosions have taken place inrecent years and it has become apparent that reliance on the presentemergency shut down procedures is insuflicient for the protection ofproperty and lives.

To explain the reason why water leakage can lead to an explosion, assumethat a leak develops in one of the tubes 12 or 15 and that water fallsdown onto the hot char bed. The chemical reaction between water and thechar bed and its smelt chemicals is complex, but for the presentpurposes, the important aspect of the reaction is that the falling watertends to form a local black spot on the char bed, that is an area ofreduced temperature, and under these conditions the water and sodiumsulphide react to form sodium sulphate and hydrogen. It is this hydrogenwhich becomes mixed with air above the char bed to form an explosivemixture.

Normal operating procedure calls for a pressure slightly belowatmospheric in the vicinity of the char bed. In other words, the induceddraft fan 30 is arranged to gether with the natural tendency of the hotgases to rise, to exceed in pressure drop the pressure gain from theforced draft fan 20. This is the preferred mode of operation, but cannotalways be maintained. Often the first indication that a water leakexists is a tendency for the pressure in the boiler in the vicinity ofthe char bed to rise slightly, such increased pressure being due tothegeneration of steam from the leaking water and the resulting excess ofvapor in the boiler. There are, of course, other ways in which a leakmay be detected, either by a drop of pressure in the Water system itselfor by visual or aural detection.

Present operating procedure provides that once the existance of a leakhas been detected, the situation is care fully watched by the operatingengineer and efforts are made to prevent the pressure at the char bedrising excessively. For example, the dampers on the forced draft fan 20may be partly closed, to reduce the input of air to the lower end of theboiler. It is not uncommon for boilers of this type to continue tooperate for weeks with one or more small leaks, until a routinemaintenance shut down becomes due. However, as soon as a leak shows anytendency to become too large, the crew initiates the shut downprocedure. Depending upon the seriousness of the leak, this may take theform of either the emergency or the normal shut down procedure. Asalready explained, the normal shut down procedure consists of turningoil? the supply of black liquor while returning the oil burners to theboiler to slowly burn away the char bed. The burners are adjusted tolower the boiler temperature gradually until it is safe to turn theburners off altogether and allow the assembly to cool down toatmospheric temperature.

The emergency shut down procedure as conventionally practiced isessentially designed to lower the temperature in the boiler as quicklyas possible, in other words to put the fire out with all possibledispatch, while of course maintaining the boiler as far as possible in acondition that is least likely to lead to an explosion. The black liquorflow is stopped, and the forced draft air flow is controlled by damperseither to be reduced or more often turned off completely. At thediscretion of the engineer the oil burners may be employed to a limitedextent at this stage in an effort to pr event the fire cooling down tooquickly, especially at local areas. Also the black liquor line may bereopened for a minute or so in an effort to form an insulating blanketover the char bed. However, once the emergency shut down procedure hasbegun in earnest, no oil and no black liquor will normally be fed intothe boiler and the forced draft damper Wlll be fully closed, the basicaim being to cool down the char bed as soon as possible. In a short timethe char bed will go completely black, the temperature already hav ngbegun to fall significantly. Even so, the whole cooling down processtakes several hours. Meanwhile, the 1nduced draft fan is kept running inan endeavour to remove as much as possible of the potentially explosivegases that can be expected to be formed by the interaction of water withthe still hot char bed. Indeed it is at this time, 1.e. while coolingdown from normal operating temperature, that the conditions are mostconducive to the formation of substantial quantities of hydrogen. In arecent accident, it was found that the induced draft fan had lost powerat this stage, because of falling steam pressure.

Not only may black liquor be sprayed on the bed at this time to providea relatively cool insulating blanket over the molten char, but it haseven been suggested that steam be injected into the boiler at this timeto attempt to smother the fire as quickly as possible. ThlS baslcconcept of trying to lower the temperature as quickly as possible isinherent in all presently practised versions of the emergency shut downprocedure, some deta ls of which may vary between various installatlonsand 1n the light of circumstances, at the direction of the variousengineers in charge.

Notwithstanding these precautions, there have been a number of recentinstances when such emergencyprocedure have proved insuflicient.Explosive combmanons of gases, especially hydrogen and oxygen have builtup 1n the boiler While the fire was still hot enough to lgnlte them. Theresulting explosions have been costly both in lives and material.

The basic concept underlying the present invention is that any attemptto smother the fire under the emergency condition of excessive waterleakage is essentrallyrmsguided, and that, conversely, the procedureWhlch is far safer is to take steps to increase the temperature of thechar bed in order to consume this bed as quickly as possible whilekeeping the same at a temperature high enough that either no significantamount of hydrogen is produced or any hydrogen that is generated isburnt, before it has been permitted to accumulate. For example, thetemperature of a typical portion of the char bed should be raised to atleast 2000 F. and preferably more to the region of 2400 F.

The preferred manner of achieving this end will now be described inrelation to FIGURES 5 to 10, in which connection it is to be understoodthat the apparatus and methods now to be described are provided merelyas example of ways in which the foregoing broad concept of the inventionmay be practiced. Alternatives within thls broad concept also form partof the present invent on, even though some of the structure employed maydllfer in form.

In order to achieve an effective increase 1n the operating temperatureof the char bed, even in the face of the leaking water which tends tocool the char bed, 1t 1s preferable to substantially prevent any excessof an enterlng the boiler (through the primary, secondary and tertiaryports) and to provide burners for generating intensely hot flames andfor supplying oxygen to the char bed, these burners being locatedadjacent the boiler floor structure in order to direct the hot flamesand oxygen directly at the char bed and particularly at the lower partthereof below and at the level of the primary ports. By reason of theintensely hot flames and oxygen directed against the char bed the sameis quickly consumed (no additional black quor being supplied) in a muchquicker time than is normal with the presently practiced routine oremergency shut down procedures.

FIGURE 9 illustrates generally a typical form of burner that could beused for this purpose. The burner itself is conventional, but itsapplication at this particular location in a recovery boiler is novel.The burner consists of an oil supply line 50 which feeds to an atomizermechanism 51 by which the oil fuel is jetted from head 52 towards thechar bed. Oxygen is supplied through pipe 53 to an annular header 54from which it issues at orifices 55 to ensure an oxygen-enrichedatmosphere in the vicinity of the char bed. Louvres 56 can be adjustedto admit some air, if desired, or to exclude all air from the device.

According to the preferred form of the present inven tion the boilerstructure is modified to introduce a number of such burners 49 into thewalls of the enclosure at approximately the level of the primary ports.Typical location of these burners 49 is indicated in FIGURES 5, 7 and 8,in which views the burners themselves are merely shown diagrammatically.FIGURE 8 also shows (purely symbolically and by way of example) thateach of the primary ports 26 is provided with a movable damper 60 bywhich such port can be fully closed. Simultaneous operation of thedampers 60 is provided for by a mechanism such as the rod 61 and controllever 62. Similar mechanisms are fitted to the secondary and tertiaryports. FIGURE 5 indicates levers 62 are controlling louvres on theprimary and secondary ports. A similar lever 62a is provided forcontrolling closure of the tertiary ports 28. It will be appreciatedthat the specific construction of these parts may differ Widely frominstallation to installation, especially having regard to the particularproblems presented at each installation by the location of other parts.The issue of importance from the point of view of the present inventionis that each of the air admission ports should :be capable of beingclosed when the burners 49 are in operation.

It is anticipated that the burners 49 would normally form part of thepermanent structure of the boiler, rather than being constructed forinsertion and withdrawal in the manner of the conventional oil burners42 (which may still be employed for normal operation). On the otherhand, if preferred in any particular installation, the burners 49 may bemade capable of being withdrawn from the boiler, with appropriateprovision then being made for closing the ports through which theyoperate, for example a downwardly slidable damper 57 as shown in FIGURE9.

The additional apparatus provided by the present invention is designedprimarily for use during emergency conditions, and specifically toenable a new and improved emergency shut down procedure to be employed.Alternatively, the burners 49 may entirely replace the con ventionalburners 42 and be used to perform the function of the burners 42 duringthe firing up process, in which case the additional apparatus will beused both during normal and during emergency operating conditions.Nevertheless, its prime purpose is to make possible the new and improvedshut down procedure which consists essentially of increasing thetemperature of the char bed, rather than decreasing it, as has been thepractice heretofore. This new procedure enables the char bed to beconsumed in a relatively short time (typically half an hour) underconditions which are not conducive to the formation of explosive gasesin any dangerous quantity. Once the char bed has been consumed theburners 49 are shut down.

When a boiler is operated under this new emergency shut down procedure,oil is supplied under pressure in each of the lines 50 and oxygen ineach of the lines 53. It is important to stress that the particular formof burner shown in FIGURE 9 is merely illustrated by way of example.Numerous suitable burners are commercially available, the importantconsideration being that an extremely hot consuming flame be generated,a result best achieved by the use of added oxygen supplied from aconvenient source, plus the necessary fuel for the burner which may beoil, or could be gas. The supplying of oxygen to these burners makesthem independent of the environmental conditions in the boiler. Therewill always be a flame, and will not be extinguished by water. Suchburners throw a short, hot flame and are arranged low down in the boiler(at or adjacent the level of the primary ports) with their flamesprojected directly towards the centre of the boiler enclosure to consumethe char bed as quickly as possible. As the char bed is consumed theburners may be adjusted to increase the flame length and project furtherinto the centre of the char bed. Any excess air entering the boiler iskept to a minimum by shutting (or, if preferred in a particular case,nearly shutting) all the dampers 60 on the normally open primary,secondary and tertiary ports, these latter being capable of normaloperation during normal operation of the boiler simply by fully openingthe dampers 60. If preferred some of the dampers can be separatelycontrolled.

The induced draft fan 30 is run at full speed during the emergency shutdown procedure, for which purpose such fan should be powered from asource independent of the steam pressure available in the "boiler atthis time. The force draft fan 20 is normally shut off, although itscondition will not normally be critical, sinc the air admission portswill be closed. In the event that the engineer in charge decides toadmit a small quantity of air to the boiler through the ports, it may bedesirable to maintain the force draft fan running at low speed. It willbe appreciated that the degree of severity of the leak and the pressureconditions resulting therefrom may call for variations in the details ofthe emergency shut down procedure at the discretion of the engineer incharge. For example, it might be that in a particular boiler it is found:better merely to close the primary and secondary ports and leave openor partly open the tertiary air ports. Nevertheless, it will bedesirable to provide some dampers or other means for closing at leastthe primary air ports and preferably also the secondary and tertiary airports, 'so that the engineer in charge has full control over any smallamount of excess air that he may decide to permit to enter the boiler.The reason for possibly permitting a certain amount of air to enter theboiler (for example through the burner louvres 56) is that it is desiredat all times during the new emergency shut down procedure to maintain asteady current of gases travelling upwardly in the boiler (principallyinduced by the induction fan 30) to remove from the interior of theboiler any small amounts of potentially explosive gases that may begenerated and to prevent their accumulating.

As a further and preferred feature of the present invention, it isconvenient to insert one or more ignitors 70 to project into the boiler,preferably at or in the vicinity of the level of the tertiary ports 28.The location of an ignitor is shown in FIGURE 5, and a larger scale viewof the device itself is shown in FIGURE 10. Like the burners 49,ignitors of this type are standard items available commercially. Mostemploy gas and have suitable provision for igniting the gas andprojecting the flame through orifices 71 at the end of the ignitor 70which projects into the interior of the boiler enclosure. In addition tothe gas fuel each ignitor is supplied with oxygen so that it cannot beextinguished by water or by lack of oxygen. Thus even under the mostadverse conditions there will always be a flame present to burn off anyinflammable gases. The fuel gas used in the ignitors should be capableof burning regardless of the ambient temperature. The ignitor orignitors thus ignite any combustible gas that may have risen from thechar bed and thus further discourage the accumulation of any such gasesin explosive quantities. Alternatively, or additionally, one or morefurther such ignitors may be inserted into the boiler at the level ofthe black liquor spray.

If desired, such ignitors once provided in accordance with the preferredform of the present invention may be used at all times, not only duringthe new emergency shut down procedure. For example they could :be usedduring the firing up procedure, during normal operation, or during thenormal shut down procedure.

I claim:

1. In the operation of a recovery boiler of the type employed in thesulphate pulp process, normal operation of said boiler comprisingforming a burning char bed in a vertically extending closure having airadmission ports, including a draft flowing upwardly in said enclosure todraw gases out of an upper part thereof and feeding black liquor ontosaid char bed to maintain the same at an elevated temperature; a methodof effecting emergency shut-down including performing substantiallysimultaneously the steps of:

(a) elevating the temperature of substantially the whole of the surfaceof said char bed to a temperature substantially above said elevatedtemperature,

(b) halting feeding of the black liquor to allow the char bed to beconsumed without replacement,

(c) maintaining an oxygen'enriched atmosphere in the vicinity of saidchar bed to encourage the full combustion thereof and to discourageliberation of combustible gases, and

(d) maintaining said upwardly flowing draft.

2. A method according to claim 1 wherein said char bed surface iselevated to a temperature of approximately 2,400" F.

3. A method according to claim 1 wherein oxygen is supplied to said charbed during said emergency shut down.

4. In the operation of a recovery boiler of the type employed in thesulphate pulp process, the normal operation of said boiler comprisingforming a burning char bed at a temperature within the range of 1200 to1800" F., and maintaining said burning char bed by feeding black liquorthereto, while confining said char bed in an enclosure, includinginducing in said enclosure a draft of gases flowing upwardly from thechar bed to be exhausted from an upper part of said enclosure; a methodof effecting emergency shut down including performing substantiallysimultaneously the steps of:

(a) elevating the temperature of substantially the whole of the surfaceof -said char bed to at least 2000 F.,

(b) halting feeding of the black liquor to allow the char bed to beconsumed without replacement,

(c) maintaining an oxygen-enriched atmosphere in the vicinity of saidchar :bed to encourage the full combustion thereof and to discourageliberation of combustible gases,

(d) and maintaining said upwardly flowing draft.

5. The method of claim 4, wherein said oxygen-em riched atmosphere isgenerated by supplying oxygen to said char bed while restricting theflow of air thereto.

6. The method of claim 4, including providing igniting means in saiddraft above the char bed to ignite any combustible gases therein.

References Cited UNITED STATES PATENTS 1,137,780 5/1915 Moore 23482,416,462 2/ 1917 Wilcoxson 23277 2,550,670 5/1951 Dalin 23-262 JAMES H.TAYMAN, In, Primary Examiner.

1. IN TH OPERATION OF A RECOVERY BOILER OF THE TYPE EMPLOYED IN THE SULPHATE PULP PROCESS, NORMAL OPERATION OF SAID BOILER COMPRISING FORMING A BURNING CHAR BED IN A VERTICALLY EXTENDING CLOSURE HAVING AIR ADMISSION PORTS, INCLUDING A DRAFT FLOWING UPWARDLY IN SAID ENCLOSURE TO DRAW GASES OUT OF AN UPPER PART THEREOF AND FEEDING BLACK LIQUOR ONTO SAID CHAR BED TO MAINTAIN THE SAME AT AN ELEVATED TEMPERATURE; A METHOD OF EFFECTING EMERGENCY SHUT-DOWN INCLUDING PERFORMING SUBSTANTIALLY SIMULTANEOULSY THE STEPS OF: (A) ELEVATING THE TEMPERATURE OF SUBSTANTIALLY THE WHOLE OF THE SURFACE OF SAID CHAR BED TO A TEMPERATURE SUBSTANTIALLY ABOVE SAID ELEVATED TEMPERATURE, (B) HALTING FEEDING OF THE BLACK LIQUOR TO ALLOW THE CHAR BED TO BE CONSUMED WITHOUT REPLACEMENT, (C) MAINTAINING AN OXYGEN-ENRICHED ATMOSPHERE IN THE VICINITY OF SAID CHAR BED TO ENCOURAGE THE FULL COMBUSTION THEREOF AND TO DISCOURAGE LIBERATION OF COMBUSTIBLE GASES, AND (D) MAINTAINING SAID UPWARDLY FLOWING DRAFT. 